XY-522: Restrict tracker comments to allowlisted public summaries

[yvette-carlisle]

Summary

• Restrict issue_comment to the allowlisted manual_attention public summary kind.
• Render manual-attention Linear comments from structured public fields and persist needs_attention ledger records.
• Update Decodex prompts/specs and tests for rejected arbitrary/private comment attempts.

Validation

• cargo make fmt
• cargo make lint-fix
• cargo make test